Electrical filters have long been used in the processing of electrical signals. In particular, such electrical filters are used to select desired electrical signal frequencies from an input signal by passing the desired signal frequencies, while blocking or attenuating other undesirable electrical signal frequencies. Filters may be classified in some general categories that include low-pass filters, high-pass filters, band-pass filters, and band-stop filters, indicative of the type of frequencies that are selectively passed by the filter. Further, filters can be classified by type, such as Butterworth, Chebyshev, Inverse Chebyshev, and Elliptic, indicative of the type of bandshape frequency response (frequency cutoff characteristics) the filter provides relative to the ideal frequency response.
The type of filter used often depends upon the intended use. In communications applications, band-pass filters are conventionally used in cellular base stations and other telecommunications equipment to filter out or block RF signals in all but one or more predefined bands. For example, such filters are typically used in a receiver front-end to filter out noise and other unwanted signals that would harm components of the receiver in the base station or telecommunications equipment. Placing a sharply defined band-pass filter directly at the receiver antenna input will often eliminate various adverse effects resulting from strong interfering signals at frequencies near the desired signal frequency. Because of the location of the filter at the receiver antenna input, the insertion loss must be very low so as to not degrade the noise figure. In most filter technologies, achieving a low insertion loss requires a corresponding compromise in filter steepness or selectivity.
In commercial telecommunications applications, it is often desirable to filter out the smallest possible pass-band using narrow-band filters to enable a fixed frequency spectrum to be divided into the largest possible number of frequency bands, thereby increasing the actual number of users capable of being fit in the fixed spectrum. With the dramatic rise in wireless communications, such filtering should provide high degrees of both selectivity (the ability to distinguish between signals separated by small frequency differences) and sensitivity (the ability to receive weak signals) in an increasingly hostile frequency spectrum. Of most particular importance is the frequency range from approximately 800-2,200 MHz. In the United States, the 800-900 MHz range is used for analog cellular communications. Personal communication services (PCS) are used in the 1,800 to 2,200 MHz range.
Microwave filters are generally built using two circuit building blocks: a plurality of resonators, which store energy very efficiently at one frequency, f0; and couplings, which couple electromagnetic energy between the resonators to form multiple stages or poles. For example, a four-pole filter may include four resonators. The strength of a given coupling is determined by its reactance (i.e., inductance and/or capacitance). The relative strengths of the couplings determine the filter shape, and the topology of the couplings determines whether the filter performs a band-pass or a band-stop function. The resonant frequency f0 is largely determined by the inductance and capacitance of the respective resonator. For conventional filter designs, the frequency at which the filter is active is determined by the resonant frequencies of the resonators that make up the filter. Each resonator must have very low internal resistance to enable the response of the filter to be sharp and highly selective for the reasons discussed above. This requirement for low resistance tends to drive the size and cost of the resonators for a given technology.
Historically, filters have been fabricated using normal; that is, non-superconducting conductors. These conductors have inherent lossiness, and as a result, the circuits formed from them have varying degrees of loss. For resonant circuits, the loss is particularly critical. The quality factor (Q) of a device is a measure of its power dissipation or lossiness. For example, a resonator with a higher Q has less loss. Resonant circuits fabricated from normal metals in a microstrip or stripline configuration typically have Q's at best on the order of four hundred.
With the discovery of high temperature superconductivity in 1986, attempts have been made to fabricate electrical devices from high temperature superconductor (HTS) materials. The microwave properties of HTS's have improved substantially since their discovery. Epitaxial superconductor thin films are now routinely formed and commercially available.
Currently, there are numerous applications where microstrip narrow-band filters that are as small as possible are desired. This is particularly true for wireless applications where HTS technology is being used in order to obtain filters of small size with very high resonator Q's. The filters required are often quite complex with perhaps twelve or more resonators along with some cross couplings. Yet the available size of usable substrates is generally limited. For example, the wafers available for HTS filters usually have a maximum size of only two or three inches. Hence, means for achieving filters as small as possible, while preserving high-quality performance are very desirable. In the case of narrow-band microstrip filters (e.g., bandwidths of the order of 2 percent, but more especially 1 percent or less), this size problem can become quite severe.
Though microwave structures using HTS materials are very attractive from the standpoint that they may result in relatively small filter structures having extremely low losses, they have the drawback that, once the current density reaches a certain limit, the HTS material saturates and begins to lose its low-loss properties and will introduce non-linearities. For this reason, HTS filters have been largely confined to quite low-power receive only applications. However, some work has been done with regard to applying HTS to more high-power applications. This requires using special structures in which the energy is spread out, so that a sizable amount of energy can be stored, while the boundary currents in the conductors are also spread out to keep the current densities relatively small. This, of course, means that resonator structures must be relatively large.
To our knowledge, the most high-power HTS resonator structures to date use circular disk-shaped resonators operating in a circularly symmetric mode, such as TM010. Some use resonators consisting of a cylindrical, dielectric puck with HTS on the top and bottom surfaces (see Z-Y Shen, C. Wilker, P. Pang, W. L. Holstein, D. Face, and D. J. Kountz, “High Tc Superconductor-Sapphire Microwave Resonator with Extremely High Q-Values up to 90K,” IEEE Trans. Microwave Theory Tech., Vol. 40, pp. 2424-2432, December 1992), while other designs just use a circular (or elliptical) disk microstrip pattern on a dielectric substrate (see K. Setsune and A. Enokihara, “Elliptic-Disc Filters of High-Tc Superconductor Films for Power-Handling Capability Over 100 W,” IEEE Trans. Microwave Theory Tech., Vol. 48, pp. 1256-1264, July 2000; K. S. K. Yeo, M. J. Lancaster, J. S. Hong, “5-Pole High-Temperature Superconducting Bandpass Filter at 12 GHz Using High Power TM010 Mode of Microstrip Circular Patch,” Microwave Conference, 2000 Asia-Pacific, pp. 596-599, 2000.) In both of these approaches the desired resonance is embedded in a fairly complex spectrum of modes, and there are other resonances that can also exist at frequencies above and below the desired resonance, some of which may be quite close in frequency to the desired resonance. Unfortunately, the lowest-frequency modes tend to have strong edge current densities, which will reduce power handling and unloaded Q values, and they are also very radiative. This causes them to interact with the resonator housing (usually composed of normal metal), which will further reduce power handling and unloaded Q values. Of course, the presence of numerous, nearby resonances in the filter response is a serious problem for many practical applications where solid adjacent stop bands are required. Thus, power handling in HTS resonators is severely limited by current density saturation.
There, thus, remains a need to provide a filter resonator that exhibits a considerable increase in power handling over that of typical HTS resonators, while having minimal unwanted mode activity and achieving very high unloaded Q's.